1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an improved top nozzle for a fuel assembly in which the coil springs of the hold-down device are advantageously replaced by a unique arrangement of leaf springs.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of helical coil springs surrounding the guide thimbles, such as seen in U.S. Pat. Nos. 3,770,583 (Re. 31,583) and 3,814,667 to Klumb et al.
Specifically, the fuel assembly of Klumb et al includes a top nozzle having an upper hold-down plate, a lower adapter plate (called an upper end plate in the patent) and a plurality of coil springs disposed between the upper and lower plates and coaxially about alignment posts which serve as upper extensions of the guide thimbles (called guide tubes in the patent). The alignment posts extend through the upper hold-down plate and are joined to the lower adapter plate and to the upper ends of the guide thimbles with fastener nuts located on the underside of the lower adapter plate. The upper hold-down plate is slidably mounted on the alignment posts and, as mentioned above, the coil springs are interposed, in compression, between the upper hold-down and lower adapter plates. A radially enlarged shoulder on the upper end of each of the alignment posts retains the hold-down plate on the posts.
The use of coil springs in the Klumb et al type top nozzle to supply the necessary hold-down force to resist upward lifting of the fuel assembly and accommodate thermal growth of the assembly presents several inherent problems.
First, coil springs inherently require considerable axial height even in applications where the required hold-down force is small. The extra amount of height required for the hold-down springs reduces the length of the fuel rods which can be used in the fuel assembly. The failure to maximize the length of the active core, as determined by fuel rod length, results in increased fuel cycle cost, increased KW per foot and increased peaking factors.
Second, coil springs are subject to coolant flow induced vibration. Cross-flow from adjacent fuel assemblies occurs because of the radial flow maldistribution across pressurized water reactor cores which is caused by core inlet flow maldistribution and by temperature differences across the core. Thus, there is a radial pressure gradient at the fuel assembly outlet which induces cross-flow above the fuel rods of the assembly. The coil hold-down springs in the Klumb et al type top nozzle are exposed to the cross-flow which has led to spring failure due to fatigue caused by flow induced vibration.
Consequently, a need exists for a different approach to the provision of adequate hold-down force in a fuel assembly top nozzle of the type disclosed in the Klumb et al patents, one with the objective of eliminating the aforementioned problems of suboptimal active core height and hold-down spring fatigue but requiring minimal modification of the overall top nozzle structure.